This invention relates to a process for the preparation of a modified 1,5-naphthylene diisocyanate ("NDI") containing urea and biuret groups.
The production of crosslinked plastics from linear polyesters or polyethers containing hydroxyl groups using organic diisocyanates in a quantity in excess of that required for reaction with the hydroxyl groups has long been known. In this reaction, the polyester or polyether chains are crosslinked by urethane groups and linear structures having free isocyanate groups at the chain ends are formed. The molecular weight of these so-called linear isocyanate polyesters or polyethers is greater when using a smaller excess of diisocyanate over the quantity required for reaction with the hydroxyl groups and vice versa. The isocyanate polyesters or polyethers thus obtained may be converted into high-quality crosslinked plastics by essentially three processes.
The first process (German Patentschrift 831,772) involves reacting the polyesters or polyethers containing isocyanate groups with glycols. In this reaction, the isocyanate polyesters or polyethers are first extended via urethane groups and in a second step additional isocyanate groups react with the urethane NH groups to crosslink the molecule via allophanate bonds. This process allows processing in the liquid phase and enables various moldings to be produced by casting without the use of solvents.
The second process uses diamines instead of glycols. The isocyanate polyester or polyether are extended via two adjacent urea groups in which the NH groups react with remaining isocyanate groups to form biuret bonds, the reaction being accompanied by crosslinking.
The third process involves reacting the polyesters or polyethers containing isocyanate groups with water so that an additional two isocyanate groups are attached by urea linkages. A high-molecular weight product is obtained in this way. Here, too, the hydrogen atoms of the ureas react with excess isocyanate groups to form crosslinking biurets. Because such biuret groups are thermally more stable than the allophanate groups described in the first process, the elastomers produced by the second or third process show better mechanical properties, as reflected in particular by their structure, elasticity, compression set, and abrasion. However, the third process has the disadvantage that carbon dioxide is given off during the reaction of the isocyanate groups with water. Consequently, the material cannot be processed in the liquid phase because of the bubbles evolved. Accordingly, processing must be carried out by a complicated method in which the foam-like polyurethane material is compression-molded under high pressure. Compare, for example, Kunststoff-Handbuch, Carl Hanser Verlag, 1966, Vol. VII, pages 270-271. As a result of the numerous individual steps involved, the process can be used only for very demanding applications. Another major disadvantage of the compression-molding step is that it is possible to produce sheeting or moldings of only very simple geometry.
On the other hand, polyisocyanate polyaddition products containing urea groups, particularly polyurethanes containing urea groups, show particularly good mechanical properties. The use of water as chain-extending agent (instead of diamines)-represents a particularly simple and inexpensive method of introducing urea groups. Accordingly, some processes are described in the patent literature in which water is used as chain-extending agent for the reaction of NCO preadducts (two-step process) or of reaction mixtures of polyisocyanates with high molecular weight and/or low molecular weight NCO-reactive compounds (one-step process). These processes are described, for example, in German Offenlegungsschriften 3,407,931 and 3,725,198 and U.S. Pat. No. 4,416,844.
A characteristic feature of this process is that the polyaddition reaction takes place in a closed mold. The carbon dioxide formed during the reaction of the polyisocyanate and water causes a very high pressure increase and, accordingly, remains in the polyaddition product in a partially or even completely dissolved form. After the time required for hardening has elapsed, the moldings may be removed from the mold without undergoing deformation caused by the dissolved CO.sub.2. Partly bubble-free solid polyaddition products are obtained, subsequently giving off the dissolved carbon dioxide gradually at room temperature. However, these processes are very complicated.
According to German Offenlegungsschrift 2,107,678, the disadvantages of the processes described above are obviated by introducing the urea groups required for biuret crosslinkage through the actual polyisocyanate, particularly 1,5-diisocyanatonaphthalene. This process is characterized by the use of a modified 1,5-diisocyanatonaphthalene containing from 0.02 to 0.5 mole (preferably from 0.1 to 0.25 mole) of urea and biuret groups per mole of 1,5-diisocyanatonaphthalene. This modified isocyanate is advantageously prepared by heating 1,5-diisocyanatonaphthalene with the corresponding quantity of tertiary alcohols, such as tert-butyl alcohol, for example, to a temperature of 130.degree. C.
Where tert-butyl alcohol is used for the preparation of the modified isocyanate, the tert-butyl urethane of the 1,5-naphthylene diisocyanate initially formed is thermally cleaved with evolution of carbon dioxide and isobutene. Where catalysts, such as hydrohalic acids or salts of nitrogen-containing bases and inorganic or organic acids, are used, the cleavage temperature can be considerably reduced so that a modified 1,5-naphthylene diisocyanate of defined structure is obtained. In the corresponding reaction of this modified 1,5-naphthylene diisocyanate with a linear polyester or polyether, the NCO preadduct containing urea or biuret groups is initially formed and may then be further processed with diols, including low molecular weight or high molecular weight diols. In this phase, the urethane groups as well as the urea groups that are already incorporated may further react with excess isocyanate groups with crosslinking of the molecule. As with crosslinking by glycols or diamines, this reaction is additive. This process is thus a combination of crosslinking by water and glycols or polyols, but with the disadvantages of crosslinking by water being excluded by the urea groups preformed in the polyisocyanate. The advantages of the process over the previously known process are that the crosslinking reaction with diols, particularly with diols of relatively high molecular weight (molecular weight 500 to 6,000), takes place more quickly by virtue of the activating effect of the urea or biuret groups already present in the polyisocyanate and that the end product may therefore be demolded after only a short time. The plastics obtained in this way are rubber-elastic and have good mechanical properties comparable with those of the water-crosslinked polyurethane elastomers (that is, the third process).
A disadvantage of this process, however, is that tertiary alcohols (preferably tert-butyl alcohol) are used as "water donors". The tert-alkyl urethanes initially formed by reaction with the isocyanate are unstable above certain temperatures, particularly in the presence of acidic catalysts, with a gas mixture of carbon dioxide and an unsaturated hydrocarbon being formed. These hydrocarbons are gaseous, flow freely, and have the disadvantage of high inflammability. For ecological reasons, however, these gases can no longer be simply "burned off". Accordingly, the hydrocarbons must be separated or isolated from the gas mixtures with carbon dioxide. This, however, involves a considerable investment in equipment.
German Offenlegungsschrift 2,107,678 discloses that, in addition to hydrogen sulfide and formic acid, water may also be used to modify 1,5-naphthylene diisocyanate. Because of the lack of further concrete disclosures, the following comparative tests were carried out.
The reaction of water and 1,5-naphthylene diisocyanate was first carried out in solvents (for example, ethyl methyl ketone, dioxane or chlorobenzene), the reaction temperatures being increased from 100.degree.to 120.degree.-130.degree. C. In every case, a solid precipitated after only a short time but did not dissolve even after a relatively long reaction time. The resultant solid was the naphthylene diisocyanate urea formed from 2 mole of 1,5-naphthylene diisocyanate and 1 mole of water. Because of its poor solubility, this compound reacts only very sluggishly with the hydroxyl-containing components. After filtration and concentration of the solvent, unchanged NDI is recovered. The NDI thus obtained contains no urea or biuret groups.
Accordingly, another test was carried out without solvent, the reaction of water with NDI (0.1-0.4 mole of water per mole of NDI) being conducted above the melting temperature of NDI (130.degree.-140.degree. C.). A considerable portion of the water was found to condense in the cooling system or on the relatively cold glass walls of the reaction vessel. As a result, this portion of water does not participate in the reaction with NDI. Consequently, the quantity of water cannot be measured exactly. The reaction is further complicated by the increasing deposition of NDI on the relatively cold glass walls caused by the pronounced tendency of NDI to sublime. NDI crusts and NDI-water secondary reaction products, such as NDI polyureas, for example, are immediately formed. This process is also unsuitable for the modification of NDI.
Accordingly, the problem addressed by the present invention is to provide an industrially simple process for the production of modified 1,5-naphthylene diisocyanate containing urea and biuret groups.
It has now surprisingly been found that NDI may readily be modified on an industrial scale if the quantity of water required for the reaction is present in a small quantity of organic solvent. The solvent should preferably have a boiling range of about 80.degree. to about 140.degree. C. and should be miscible with water or should at least form an azeotropic mixture with water in that boiling point range.